The first really exciting thing from Run 2 at the CERN Large Hadron Collider (at least for me) has finally appeared. A 2.9 TeV dielectron event was recorded by CMS on August 22. At this mass a new Z' boson is not excluded by Run 1 searches.... And in the whole Run 1 data the highest-mass dielectron event collected by CMS was only 1.8 TeV. So by raising the centre-of-mass energy by 60% we collect a 60%-higher-mass event, but with 0.5% of the collisions. It is nice to think that the event might really be the first hint of a new resonance !
Extensions of the standard model have been put forth which include new so-called "Z' resonances" as the observable result of the addition of a new group of symmetry to the one the standard model is built on. This extra "U(1)" symmetry would not per se explain a lot of whatever new physics exists out there, but it would be a very clear indication that the standard model breaks down at the energy scale of a few TeV and that it must be replaced by something more complex -at least apparently.
A Z' is a boson like the Z, the particle discovered by Rubbia's UA1 experiment in 1983 and studied in excruciating detail by the LEP collider in the nineties. The Z has a mass of 91 GeV, and is itself the "remnant" of a U(1) group of symmetry which is part of the origin of the electroweak interactions. If a Z' exists, new interactions exist, and particle physicists will have a new world to explore in the forthcoming years.
The event display of this spectacular collision is shown below. As you can see, there is very little extra activity in the event besides the very energetic electron and positron.
Above is a 3-D view of the CMS detector, with red and blue boxes indicating energy deposits in the calorimeter (respectively, electromagnetic and hadronic deposits) and green and blue tracks showing particle paths traced in the silicon detector. The two electron tracks are in blue.
Above you can see the transverse view of the event, with the two impressively back-to-back electrons in evidence.
Some more data on the event: The two objects are perfectly balanced (1260 and 1280 GeV of transverse energy). The expected backgrounds for M(ee)> 2.5 TeV amount to 0.002 events, but their estimate has not been finalized yet. The dielectron rapidity is largish.
Now, I would caution anybody from rushing to a Poisson calculator to evaluate the probability that one event is seen when 0.002 are expected. The number you would get (p=0.002, or a bit less than three sigma) would be rather meaningless, as we do not know the shape of systematic uncertainties out there. Still, the event remains awe-inspiring and I intensely hope that we'll see others pile up in the thereabouts as we continue collecting Run 2 data at 13 TeV. As we have gotten one event in 100 inverse picobarns of data or so, it is reasonable to expect to see more similar events in the forthcoming weeks - unless we have been very "lucky" with this first batch of data. So stay tuned!
A 3 TeV Dielectron Event By CMS !
By Tommaso Dorigo | September 3rd 2015 03:56 AM | Print | E-mail